[PATCH v2 net-next 6/8] net/sched: mq: no longer acquire qdisc spinlocks in dump operations

[Eric Dumazet <edumazet@google.com>]

Prepare mq_dump_common() for RTNL avoidance.

Use RCU instead of RTNL, and no longer acquire each children spinlock.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 include/net/sch_generic.h |  9 +++++++++
 net/sched/sch_mq.c        | 32 ++++++++++++++++++++++----------
 2 files changed, 31 insertions(+), 10 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 2ce84b2c9423ee3b945c0a537620b2568279e1ca..9e6ed92729d282642e17a72cc578f25e1d22e4d9 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -952,6 +952,15 @@ static inline void _bstats_update(struct gnet_stats_basic_sync *bstats,
 	u64_stats_update_end(&bstats->syncp);
 }
 
+static inline void _bstats_set(struct gnet_stats_basic_sync *bstats,
+			       u64 bytes, u64 packets)
+{
+	u64_stats_update_begin(&bstats->syncp);
+	u64_stats_set(&bstats->bytes, bytes);
+	u64_stats_set(&bstats->packets, packets);
+	u64_stats_update_end(&bstats->syncp);
+}
+
 static inline void bstats_update(struct gnet_stats_basic_sync *bstats,
 				 const struct sk_buff *skb)
 {
diff --git a/net/sched/sch_mq.c b/net/sched/sch_mq.c
index 4172ec24a43d1c2fe56789986a46da93eb522721..5a6b14557d85fdc6c2462ee9e926155438f597b8 100644
--- a/net/sched/sch_mq.c
+++ b/net/sched/sch_mq.c
@@ -143,30 +143,42 @@ EXPORT_SYMBOL_NS_GPL(mq_attach, "NET_SCHED_INTERNAL");
 void mq_dump_common(struct Qdisc *sch, struct sk_buff *skb)
 {
 	struct net_device *dev = qdisc_dev(sch);
+	struct gnet_stats_queue qstats = { 0 };
+	struct gnet_stats_basic_sync bstats;
+	const struct Qdisc *qdisc;
 	unsigned int qlen = 0;
-	struct Qdisc *qdisc;
 	unsigned int ntx;
 
-	gnet_stats_basic_sync_init(&sch->bstats);
-	memset(&sch->qstats, 0, sizeof(sch->qstats));
+	gnet_stats_basic_sync_init(&bstats);
 
 	/* MQ supports lockless qdiscs. However, statistics accounting needs
 	 * to account for all, none, or a mix of locked and unlocked child
 	 * qdiscs. Percpu stats are added to counters in-band and locking
 	 * qdisc totals are added at end.
 	 */
+	rcu_read_lock();
 	for (ntx = 0; ntx < dev->num_tx_queues; ntx++) {
-		qdisc = rtnl_dereference(netdev_get_tx_queue(dev, ntx)->qdisc_sleeping);
-		spin_lock_bh(qdisc_lock(qdisc));
+		qdisc = rcu_dereference(netdev_get_tx_queue(dev, ntx)->qdisc_sleeping);
 
-		gnet_stats_add_basic(&sch->bstats, qdisc->cpu_bstats,
-				     &qdisc->bstats, false);
-		gnet_stats_add_queue(&sch->qstats, qdisc->cpu_qstats,
+		gnet_stats_add_basic(&bstats, qdisc->cpu_bstats,
+				     &qdisc->bstats, true);
+		gnet_stats_add_queue(&qstats, qdisc->cpu_qstats,
 				     &qdisc->qstats);
 		qlen += qdisc_qlen_lockless(qdisc);
-
-		spin_unlock_bh(qdisc_lock(qdisc));
 	}
+	rcu_read_unlock();
+
+	spin_lock_bh(qdisc_lock(sch));
+	_bstats_set(&sch->bstats, u64_stats_read(&bstats.bytes),
+		    u64_stats_read(&bstats.packets));
+	spin_unlock_bh(qdisc_lock(sch));
+
+	WRITE_ONCE(sch->qstats.qlen, qstats.qlen);
+	WRITE_ONCE(sch->qstats.backlog, qstats.backlog);
+	WRITE_ONCE(sch->qstats.drops, qstats.drops);
+	WRITE_ONCE(sch->qstats.requeues, qstats.requeues);
+	WRITE_ONCE(sch->qstats.overlimits, qstats.overlimits);
+
 	WRITE_ONCE(sch->q.qlen, qlen);
 }
 EXPORT_SYMBOL_NS_GPL(mq_dump_common, "NET_SCHED_INTERNAL");
-- 
2.54.0.563.g4f69b47b94-goog